Many techniques are available for solution separation. In recent years, membrane separation has been in wider use because it requires smaller energy and smaller resources. Microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO) are among the membrane separation techniques. More recently, loose RO or nanofiltration (NF), whose functions are between those of reverse osmosis and ultrafiltration, has come in use. Reverse osmosis, for example, is currently used for desalination of sea water or brakish water to provide water for industrial, agricultural and household uses. With reverse osmosis, a pressure higher than the osmotic pressure is exerted on salt water to allow it to permeate reverse osmosis membrane to obtain desalted water. This technique can produce drinking water from sea water, brine, or a water which contains harmful substances, and has been used for the preparation of ultra-pure water for industrial use, treatment of waste water, and recovery of useful materials.
The production of fresh water from sea water by reverse osmosis has the advantage that it involves no phase transition such as found in evaporation. In addition, it requires less energy and less operation maintenance, resulting in its wider use in recent years.
For separation of a solution by reverse osmosis, it is necessary to supply a solution to the reverse osmosis membrane with a pressure larger than the chemical potential (which can be expressed in terms of osmotic pressure) of the solution which depends on the content of the solute in the solution. When a reverse osmosis membrane module is used for separation from sea water, for example, a pressure above 30 atm, or more practically a pressure above 50 atm, is required. Sufficient reverse osmosis separation performance cannot be obtained at pressures lower than this.
Concerning sea water desalination through reverse osmosis membrane, for example, permeable sea water recovery of conventional sea water desalination is not more than 40%. The concentration of sea water in the reverse osmosis membrane module increases from 3.5% to about 6% as a volume of fresh water equal to 40% of the supplied sea water is obtained through the membrane. A pressure larger than the osmotic pressure corresponding to the concentration of the concentrate (45 atm for 6% sea water concentrate) is required to achieve permeate water recovery ratio of 40%. Practically, a pressure about 20 atm larger than the osmotic pressure that corresponds to the concentrate concentration (which is called the effective pressure) is necessary to produce a sufficient fresh water that can be used as drinking water. Thus, reverse osmosis membrane separation for desalination of sea water have been conventionally operated under a pressure of 60-65 atm to achieve a recovery ratio of 40%.
A higher permeate water recovery (recovery ratio) is more desirable since the recovery ratio directly affects the required cost. Conventionally, however, there have been limits to recovery ratio improvement. That means, an increased recovery ratio may require a very high pressure. In addition, as the concentration of sea water components increases and in higher recovery ratio operating conditions, the contents of scale components such as calcium carbonate, calcium sulfate, strontium sulfate and other salts deposits on the reverse osmosis membrane as scale to cause clogging.
At the recovery ratio of about 40% (which is now widely recognized as the practically maximum recovery ratio), it is unlikely that such scale may be formed in significant amount and therefore no special means are required against them. If an attempt is to be made to operate reverse osmosis separation at a higher recovery ratio, a scale prevention agent that increases the solubility of salts should be added in order to control the deposition of these scale components. Despite the addition of such a scale prevention agent, however, the control of the deposition of said scale components is effective only to increase the concentrate concentration by 10-11 percentage points. For the desalination of sea water of a salt concentration of 3.5%, a mass balance analysis indicates a limit recovery ratio of 65-68%.
Taking into account the effects of various other components of sea water, the practical limit of recovery ratio at which a reverse osmosis sea water desalination plant can be operated stably would be about 60%.
In a practical sea water desalination process, a pressure about 20 atm higher than the concentrate's osmotic pressure should be applied on the reverse osmosis membrane, as stated above. When the salt concentration in sea water is assumed to be 3.5% and a recovery ratio of 60%, the concentration of salt becomes 8.8%, which corresponds to an osmotic pressure of about 70 atm. Thus, a pressure of about 90 atm has to be applied to the reverse osmosis membrane.
For practical uses, several reverse osmosis elements connected in series are loaded in a pressure vessel, which is called a module, and many modules are installed in parallel in a practical plant. The recovery ratio of a sea water desalination plant is defined as the ratio of the total water permeation to the total sea water supplied to the reverse osmosis modules. In an ordinary plant, since modules are installed in parallel, the recovery ratio is equal to the ratio of the desalted water obtained from a module to the sea water supply to that module. In case that one module contains six reverse osmosis membrane elements and that 198 m.sup.3 /day of sea water is supplied to the module to produce 78 m.sup.3 /day of desalted water (40% recovery ratio), a simulation shows that 18-19 m.sup.3 /day and 15-17 m.sup.3 /day of desalted water comes from the first and second elements, respectively, followed by decreasing amounts from the remaining elements to produce a total 78 m.sup.3 /day of desalted water. Thus, in total, desalted water is obtained from the entire module at 40% recovery ratio despite a small desalted water recovery ratio for each element.
Prevention of fouling and concentration polarization (localization of solute) is an important factor to be considered in establishing operation conditions of a reverse osmosis membrane separation process. To prevent fouling, the rate of desalted water production from one reverse osmosis membrane element should be controlled below a certain limit (fouling-resistant permissible flux). If the rate exceeds the limit, the fouling on the membrane will be accelerated to cause trouble. The fouling-resistant permissible flux for high-performance reverse osmosis membrane is generally in the vicinity of 0.75 m.sup.3 /m.sup.2 eday, which corresponds to an yield of 20 m.sup.3 /day for a reverse osmosis membrane element with a membrane area of 26.5 m.sup.2 (the membrane area of an element is assumed to be 26.5 m.sup.2 in all calculations hereinafter). Thus, to prevent fouling, the desalted water production rate of an element should be controlled below 20 m.sup.3 /day.
The rate of water supply to elements in module decreases as water flows from upstream elements to downstream ones. Concentration polarization referred to above is caused due to a decrease in the flow rate of supplied water through the membrane in the final element. Concentration polarization not only reduces the membrane performance but also accelerate fouling to shorten the life of the reverse osmosis membrane element. To prevent this, the flow rate of the concentrate in the final element (with a membrane area of 26.5 m.sup.2) should be maintained above about 50 m.sup.3 /day.
When a reverse osmosis membrane sea water desalination plant is to be operated at the conventional maximum recovery ratio of about 40%, the above-mentioned fouling and concentration polarization prevention conditions can be easily met and operation can be performed stably by simply arranging several modules in parallel, applying a pressure of 65 atm (when the temperature is 20.degree. C.), and setting the water supply rate to 2.5 times the final desalted water production rate. It is not necessary to give special consideration to the balance among the flow rates or the deposition of scale in the elements in each module.
An increased recovery ratio, however, is essential to further reduce the cost of sea water desalination process by the reverse osmosis membrane. As described above, its increase up to 60% is desired for desalination sea water with a salt concentration of 3.5%. After adding an appropriate amount of a scale prevention agent, the plant have to be operated at 90 atm, which is 20 atm higher than the osmotic pressure of the concentrate.
Scale prevention agents have been used in some reverse osmosis membrane apparatus like those for water processing plants and sea water desalination apparatus that uses evaporation. They are designed, however, mainly for controlling the deposition of such scale components as silica and metal salts within the apparatus. In particular, such agents haven been used widely to treat water with a high silica scale content.
For example, Japanese Patent Laid-Open (Kokai) SHO53-30482 proposes that the life of reverse osmosis membrane can be lengthened when reverse osmosis operation is performed after the contents of calcium, magnesium etc. are reduced by allowing the supply water to make contact with chelate resin. Japanese Patents Laid-Open (Kokai) SHO52-151670 and HEI 4-4022 disclose a method in which a phosphate is added to prevent the formation of scale in reverse osmosis apparatus. Japanese Patents Laid-Open (Kokai) SHO63-218773 and HEI 4-99199 and Japanese Patent Publication (Koho) HEI 5-14039 propose a method in which waste water from electrodeposition coating and copper plating processes is concentrated by subjecting it to reverse osmosis membrane treatment after adding a chelating agent to recover coating material and copper. Furthermore, Japanese Patents Laid-Open (Kokai) SHO63-69586 and HEI 2-293027 disclose that sterilization and stable operation of reverse osmosis membrane apparatus can be achieved by supplying a solution that contains chlorine or a mixture of a oxidizer and a phosphate.
However, if, as in conventional apparatus, several reverse osmosis membrane elements are placed in series in a pressure vessel to produce a module, and a pressure of 90 atm is applied to several such modules arranged in parallel to achieve a desalted water recovery ratio of 60%, then the flow rate of treated water from the upstream elements (first and second elements) in each module will exceed the permissible value to cause concentration polarization and fouling in these elements, leading to clogging and reduction in the life of the elements. As a result, it would become very difficult to operate the reverse osmosis membrane apparatus stably for a long period of time. In a sea water desalination process operated at an recovery ratio of 60%, the salt concentration and osmotic pressure vary from 3.5% to 8.8% and from 26 atm to 70 atm due to material balance requirements as the water flows from the inlet to the outlet. The operating pressure, on the other hand, is nearly constant over the entire process from the inlet to the outlet, indicating that the effective pressure required for the permeation of desalted water (i.e. the difference between the operating pressure and the osmotic pressure) varies largely from 64 atm to 20 atm. The ratio of permeation through the first element to that through the final element in the same module is of the order of the ratio of effective pressure, i.e. 64:20. Thus, in conventional plants, the permeation rate in the first element can undergo a sharp increase to allow the total permeation rate to exceed largely the fouling-resistant permissible limit of 20 m.sup.3 /day, which means that fouling is caused very easily. However, it is impossible to decrease the operating pressure because an operating pressure of 90 atm is essential to the achievement of an recovery ratio of 60%. This indicates that operation at an recovery ratio of 60% would not be appropriate and if it is attempted despite these considerations, fouling would be accelerated and long-term stable operation would be impossible. Or, operation at a 60% recovery ratio would have to be achieved under very costly operating conditions which may include the use of a large number of low-performance elements with a decreased permeation rate.
Spiral type reverse osmosis membrane elements are considered to make the matter simple in the above description. However, the same phenomena and problems will occur also in hollow fiber membrane type modules.
The present invention provides apparatus and separation method that produces a low-concentration solution from a high-concentration solution with a high recovery ratio, small energy, high efficiency and high stability. In particular, it aims to provide apparatus and method that produces fresh water from sea water with a high 60% recovery ratio, small energy requirements, high efficiency and high stability.